Laminated panels



F IG. 4

Sm Fle.

FIG. 3

H. A. PACE LAMINATED PANELS Filed June 2l, 1951 Il J I May .1, 1956INVENTOR. H EN RY A. PA C E /KT'TORNEY FIG. 6

ments, to The Goodyear-Tire` 8aA Rubber Company, acorporation of OhioApplication JuneZl, 1951; SeralNo; 232}691i 4 Claims. (Cl. 154-75) This'invention relatesvtof the fabrication" off laminated sandwich panels andis particularly concernedlwith an* improved method for"rr'ranufa'ctui-ing`v panels wherein` a honeycomb spacer is filled` withAaE toa-med res-inf Pr'io'r practices in the' laminating!fieldhavetbeen-di'-I rected toward obtaining a combination of desirable char:acteristics, namely, maximum structural strength-coupled with' minimumdensity. In'cerltain'-usesf1for' laminated structures additionalcharacteristics'f arefessentiaL forexz ample, fire' resistance; goodelectrical properties, g'ood insulating characteristics; andir'n'pctrta'sistance:A

Various attemptslhavebelllmadt-fabicafe aStIlC: tu're embodying?th'es'efI desirableclara'eterist-ic's' Never# theless.- it has been:customary to:K sacrifice; strength. to obtainlow densityor toVsacrifc-:e` low density to'obtain maximum' strength. In: certaindesirable uses-A for larn'ifl nated structures, theincorporationoithese'optimum coni ditionslin a practical laminatedf structure"haswbe'en im'- possible-*or*impracticahas;` forexample; inthefabrication" of radar canopies;` guided-missile parts;airplanefcon'iponents,- and other specific uses.

It is, accordingly, an object of this invention--tof'provide avstructure characterized bythe presence of a'p'or'ous filler whichcombines maxim-umlfstr'ength and minim`um density.

It is another' object of thi's. invention to provide a laminate whereina teamed resin'- is-dispos`ed Withinthe interstices of a honeycombystruc-turefin. ordentofsecure minimum densityy andstillretain-greatfstrength, impact resistance, anddesirableelectrical-properties* A further object offth-is inventionis--tofabricate such a-V laminate which resists: thermal andstructuralYdistorL tion and -which has goodheat-insulating-characteristicsi Stillanother object ofthis inventionlis to provide a method for preparingv.laminated :sandwichpanelshaving superior physical characteristics.

Inthe practice of thisinventiomouter facingI-sheets, or skins,areprepared in. the customarytmarmer.V Such sheets can be made of'paper,sheet metal, Woodglass fiber laminates or other sheetedfmaterials; Theouter skin material is quite generally selected inviewof the usecontemplated for'the finished product.- Woven glass fiber laminates arepreferred'n-'the practice ofrthis' invention.

Honeycomb type spacers can be prepared'according to one of thegenerally' accepted" practices; For ex'- arnple, strips of material can'be' spot-glued,A expanded to the desired honeycomb structure, and heldin flexed position by resin impregnation. Alternatively, theymaybemolded from resin-impregnated materials or'they"may b'e constructed byshaping'at sheets o'n a'man'drel'. impregnated paper', impregnated clothWoven from glass' fibers', shaped aluminum and othersimila'r' materials'm'a-y beH employedin preparing. the'honeycomb" sp'a'cer'sused inkpracticing y this invention. Shaped` laminatedv woven fibrous glassisgenerallypreferredfin fabrieatingfpanels accordingy to` the teachingsofrtbisfinvention; The: size of the open-ended cells of the honeycomb isnot critical "nid States Patent ice 2v but a--reasonableprop'ortionbetween the size'oftliel'c'e`-llE andthe sizeof the finished-product'sliouldbe maintained. For example; the` transverse rib'sinraydividetlie 'structure intoany`vl numberl of' cellsl'actvveenx twoVH andsuch amultitude asito unduly restrain tlie ri'seof the foa'rn'.'y Bothofthese extremes should be avoided#` The foamable rnixtureshouldI bethermosettin'g" and interiorlyv cohesive' as wellI as' capable ofadli'e'siorr to' the'fspacer walls and thefacing'sheet's. In'the'practice of this invention, the use` of a'n alkydresin, blown'- by'means of a polyisocyanate; is preferred. The foa'rna-ble material canbeiplacedonlonef skin o'r facing-" sheet' to@ adepth" calculatedt givethe desired faxn'fdnfsity in th'e finished product'. Forexampl'egag'ive'ri`l foaming mixture one-fifth. the height of thehoneycornhi'spacerv will `'produce' af foam having a= density ofapproximately 10 pounds per cubic foot. This ratio is preferablypreliminarily established f'or' each' foamable mixture emplayed.

The honeycomb', or spacer elementfisplaced adjacent a skin with theopen-ended spacer cell walls in sub s'tantiauy perpendicular rel'tionete: the skin' andilwt-h' the foamable materialretained-withinthe'cellular-*-'spaces'.AA The other skinv is then' placed on topofthehoneycorli spacer. The entire assembly. is held in rigidrelationship" between'the platensof a' press' or by'm'e'a'ns' of platesand:` C clamps; The" fo'am'able' material is thfen" foar'r'fed` and'jelled by means of heat'which maybe's'uppliedby any vconventionalheating' means. It is` gen'- eral-ly desirable to: con'n'e the honeycombwith end a'nd side shims to prevent rupture' of'the'wa'lls 'drin'gthefoaming operation because" internal pressure as' hig'lita's 30 p'; s.'i. gauge may be' generated.'

In this'man'ner, a superior laminated'structureisfabri# c'ate'd' whichis characterized by 'thehigh 'tensile strength of* thefoam and 'the stron'g' bond'b'e'tween'the" foama'nd the fa'vin'gsheets. This unusual*strength' is'due p'ri' marily to lthe restrained Efoaming`whiclfcr'eates'l a finer cellular structure Within' the foam; as more'particularly setffo'rth: hereinafter. A"number ofplies may bela'nii-`nated together. simultaneouslfyin order' to* fabricateia multiplesandwich panel of even greater strength.

The invention is more particularly described below in connection withthe accompanying drawings wherein;

Figure l is an' elevation partly in' s'etio'r'of 'a/jfcin'g member with'a laye'r'of'foaniable' mixture placed thereon;

Figure 2 is a' sideelevator'i' showing' a honeycomb spacer in place; K

Figure "3*is" a View' in' elevation'showing" a'. sandwichpanel betweenthe pl'atensof a conventional'pre'ss;

Fi'gure'fI is' acut-away section;` showing' foam' inthe' course ofrising. during .the blowing operation; y

I Figure 5 is' a fragmentary'sectional Viewv of the finished laminate;and

Figure 6 is a perspectveview'ofthe finished laminate showing' afragmentary portion of the foam-filled spacer inters'tices. h

Referring tothe' drawings,` the previously prepared foamable material 1is placed'on thepr'eviously' prepared' lower facing'skin Z. A` honeycombs'tructre`3' is' then pushed through the foamable material'until it'issubstantially adjacent the' facing skin' and substantially' per'-vpendicular thereto. The upper'facing'skin 4` isA then placed in'positionover" the honeycomb spacer member' and lthe entire assembly placedbetween thev platen mem# b'er's 5 and 6 0f`a press, as shown in Figure3. Heat is thena'p'plied'to foam the material andfill the interstices ofthel spacerwith foam 7 whileholding' the facing'skins Ziand '4inparallel spaced relation.

Inthe 'practice of the-invention; theupperfacng skin 4 'islheldiinlfixed relatio'ris'hip-v with-theI sp'cer elemetfiS'; but loosely enoughto allow the escape of entrapped air reventa Msyr, lisse as Vthe-foamrises in the interstices. Likewise, confining side shims or side boards,8, 9 and 10, shown in Figure 3, and the fourth side shim, which is notshown, are held in fixed relation to the facing sheets of the laminate,but loosely enough to allow the escape of entrapped air. These shims arefixedly connected to each other so as to confine the foam laterally toprevent rupture of the cell walls during the foaming operation. As thefoam rises during gas evolution, the spaces between the honeycomb andthe facing sheets and the spaces between the confining shims and thefacing sheets become filled with foam, thus preventing escape of gas andcreating a restraint on the foam while the blow is being completed. Thisprocedure makes for a finer, more uniform cell structure and createsgood cohesion within the foam as well as good adhesion betweenr'the foamand the facing skins.

The final result is a structure having greater strength for the samedensity than related structures heretofore manufactured.

The invention is illustrated by the following examples:

Example 1 A foaming mixture was prepared by mixing together thefollowing materials:

Glycerol adipate phthalate alkyd resin grams 75 2,4-tolylenediisocyanate do 70 Water cc .5

A wooden frame 5 x 5 x 3/8 and a laminated woven glass fiber honeycombstructure having sufficient free space to hold a calculated amount offoam were made ready. The foamable mixture was poured upon a previouslyprepared sanded glass fiber skin within the confines of the frame, andallowed to level out. The previously warmed honeycomb was then pressedinto place. A second sanded glass fiber skin was placed upon thehoneycomb and the assembly was clamped together between reinforcedalurninum plates. The foamable material was then blown by heating forone hour at a temperature of 150 F. followed by two hours at 275 F.

A cross `section through the laminated panel prepared above showed veryuniform foam structure and extremely strongadhesion between the foam andthe spacer cell walls and between the foam and the facing sheets.

Example 2 Homogeneous mixture of unsaturated alkyd resin and styrene ina ratio of about 1 part Grams resin to 2 parts styreue A 60 2,4-Tolylenediisocyanate 9 Benzoyl peroxide 2 Water 1 Sodium lauryl sulfonate 0.2

The alkyd resin was prepared by reacting fumarie or maleic acid andglycol. A sandwich panel was fabricated from the foamable mixture as inExample l. This panel likewise showed uniformity of cell structure,strong cohesive strength within the foam, excellent adhesion between thefoam and the structure elements and exhibited good heat-insulationqualities.

The invention may be practiced with various alkyd resins, these beingthe condensation products of polyhydric alcohols and polycarboxylicacids. Representative polyhydric alcohols which can be used in preparingalkyd resins for use in the practice of this invention are ethyleneglycol, triethylene glycol, propylene glycol, dipropylene glycol,hexamethylene glycol, pinacol, and glycerol. Representativepolycarboxylic acids which can be used in preparing alkyd resins byreaction with any of the above-mentioned polyhydric alcohols and othersare phthalic, isophthalic, terephthalic, succinic, adipic, sebacic,maleic, fumarie, 2-phenylglutaric, citric, citraconic, mesaconic,itaconic, oxalic, malonic, glutaric, pimelic, suberic, azelaic andsebacic acids. It is possible to substitute ester-forming derivatives,such as the anhydrides, the simple esters and the like for the acidsthemselves. Also certain monofunctional modifiers can be incorporated inthe resin according to the known art. ln order to have sufficientcarboxyl radicals available to react with the isocyanates, it isdesirable to keep the acid number of the alkyd resin within a range of35 to 45.

In the practice of this invention, foaming is obtained by reacting anisocyanate with the carboxyl groups of the alkyd resins and with anywater which may be present in the resin. The isocyanate employed may bealiphatic or aromatic and in order to facilitate mixing should be liquidat normal temperatures. Representative isocyanates which can be used inpreparing the foamable mixture are 2,4-tolylene diisocyanate,hexamethylene diisocyanate, diphenylmethane diisocyanate, etc.

While certain representative embodiments and details have been shown forthe purpose of illustrating the invention, it will be apparent to thoseskilled in this art that various changes and modifications may be madetherein Without departing from the spirit or scope of the invention.

I claim: y

1. The method of preparing a laminated sandwich structure comprising thesteps of (l) placing a thin layer of a foamable mixture prepared from afoamable alkyd resin having an acid number of 35 to 45 and an isocyanateblowing agent on a facing sheet and pressing a rigid uniform honeycombspacer element embodying a plurality of open-ended cells into thefoamable mixture, (2) placing another facing sheet on top of the spacerelement and confining the sandwich structure between limiting platenmembers, and, (3) completing the sandwich structure by expanding thefoamable mixture until the interstices of the spacer element aresubstantially completely filled with foam and the spacer Walls areadhesively joined thereby to each other and to the facing sheets.

2. The process according to claim l wherein a plurality of plies aresimultaneously foamed together. Y

3. The method of preparing a laminated structure comprising the steps of(l) placing a foamable mixture between a pair of confining laminae andwithin the interstices of a honeycomb spacer element, said foamablemixture filling about one-fifth of the volume contained between thelaminae and being comprised of an alkyd resin having an acid number of35 to 45 and a liquid isocyanate blowing agent, said honeycomb spacerelement embodying a plurality of uniform open-ended cells' positioned insubstantially perpendicular relation to the confining laminae and inadjacent relation thereto, and (2) heating said foamable mixture at atemperature of F. to 275 F. to decompose the isocyanate and to cause thealkyd resin to expand within the interstices of the honeycomb spacerelement until said interstices are completely foam filled and theconfining laminae are adhesively joined by the alkyd resin foam.

4. The method of preparing a laminated structure comprising the steps of(l) placing a foamable mixture betweene a pair of confining laminae andwithin the interstices of a honeycomb spacer element, said foamablemixture lling about one-fifth of the volume contained between thelaminae and being comprised of an alkyd resin having an acid number of35 to 45 and a liquid diisocyanate blowing agent, said honeycomb spacerelement embodying a plurality of uniform open-ended cells positioned insubstantially perpendicular relation to the confining laminae and inadjacent relation thereto, and (2) heating said foamable mixture at atemperature of 150 F. to 275 F. to decompose the diisocyanate and tocause the alkyd resin to expand within the interstices of the honeycombspacer element until said interstices are completely foam filled and theconning laminae are adhesively joined by the alkyd resin foam.

References Cited in the le of this patent UNITED STATES PATENTS ThomsonJan. 9, 1934 Budd et al Oct. 6, 1936 Boyer May 22, 1945 Staudinger etal. June 8, 1948 Rheinfrank lan. 3, 1950 Moxness June 20, 1950 Kropa etal Nov. 20, 1951 Simon et al. Dec. 4, 1951 Simon et al. July 8, 1952Simon et al. May 19, 1953 FOREIGN PATENTS Great Britain May 29, 1946Great Britain May 31, 1946 OTHER REFERENCES German Plastics Practicepublished by DeBell and Ricardson, Springeld, Mass. in 1946; pp.463-465.

Isocyanate-Based Adhesives article published by

1. THE METHOD OF PREPARING A LAMINATED SANDWICH STRUCTURE COMPRISING THESTEPS OF (1) PLACING A THIN LAYOR OF A FOAMABLE MIXTURE PREPARED FROM AFOAMABLE ALKYD RESIN HAVING AN ACID NUMBER OF 35 TO 45 AND AN ISOCYANATEBLOWING AGENT ON A FACING SHEET AND PRESSING A RIGID UNIFORM HONEYCOMBSPACER ELEMENT EMBODYING A PLURALITY OF OPEN-ENDED CELLS INTO THEFOAMABLE MIXTURE, (2) PLACING ANOTHER FACING SHEET ON TOP OF THE